A curvilinear interpolation system and method for use in a numerical control apparatus such as a machine tool or a robot having two or more drive axes and movable while effecting linear interpolation on a given curve with a plurality of line segments. For each line segment a curvature Kp of the curve...http://www.google.com/patents/US4648024?utm_source=gb-gplus-sharePatent US4648024 - Curvilinear interpolation system and method

A curvilinear interpolation system and method for use in a numerical control apparatus such as a machine tool or a robot having two or more drive axes and movable while effecting linear interpolation on a given curve with a plurality of line segments. For each line segment a curvature Kp of the curve is calculated at a current curve interpolating point Pp, and a curvature Kn calculated at a point Pn on the curve spaced a given search length lS from the point Pp along the curve. Then, a curvature Ks is set equal to Kp when Kp ≧Kn and set equal to Kn when Kp <Kn. The curvature Ks is compared with a limit curvature KL established for determining whether a curve portion is a straight line or an arc for establishing the curve portion as an arc having the curvature Ks when Ks ≧KL to approximate the curve portion with a line segment so as to fall within an allowable error t and for establishing the curved portion as a straight line when Ks <KL to approximate the curve portion with a line segment of a prescribed length. An operative member such as a robot arm is then moved along a path defined by the plurality of line segments thereby calculated.

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Claims(2)

We claim:

1. A curvilinear interpolation system for use in a numerical control apparatus such as a machine tool or a robot having two or more drive axes and capable of moving a movable member while effecting linear interpolation on a given curve with a plurality of line segments, comprising:

means for calculating a curvature Kp of said curve at a current curve interpolating point Pp ;

means for calculating a curvature Kn near range of a point Pn on said curve spaced a given search length S from the point Pp along the curve;

means for setting Ks =Kp when Kp ≧Kn and setting Ks =Kn when Kp <Kn ;

means for comparing said curvature Ks with a limit curvature KL established for determining whether a curve portion is a straight line or an arc, for establishing said curve portion as an arc having said curvature Ks when Ks ≧KL to approximate said curve portion with a line segment so as to fall within an allowable error t and for establishing said curve portion as a straight line when Ks <KL to approximate said curve portion with a line segment of a prescribed length, so as to define all of said plurality of line segments; and

means for moving said movable member along a path defined by said plurality of line segments.

2. A curvilinear interpolation method for use in a numerical control apparatus such as a machine tool or a robot having two or more drive axes and capable of moving a movable member while effecting linear interpolation on a given curve with a plurality of line segments, comprising the steps of:

(1) for each line segment:

calculating a curvature Kp of said curve at a current curve interpolating point Pp ;

calculating a curvature Kn near range of a point Pn on said curve spaced a given search length S from said point Pp along said curve;

setting Ks =Kp when Kp ≧Kn and setting Ks =Kn ;

comparing said curvature Ks with a limit curvature KL established for determining whether a curve portion is a straight line or an arc for establishing said curve portion as an arc having said curvature Ks when Ks ≧KL to approximate said curve portion with a line segment so as to fall within an allowable error t and for establishing said curve portion as a straight line when Ks <KL to approximate said curve portion with a line segment of a prescribed length; and

(2) moving an operative member along a path defined by said plurality of line segments.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a curvilinear interpolation system and method for a numerical control apparatus such as a machine tool or a robot having two or more drive axes and movable while effecting linear interpolation on a given curve with a plurality of line segments.

It is difficult to numerically control a machine tool or robot so as to move without error along a curve defined in a two- or three-dimensional space. Therefore, it has been the customary practice to approximate the curve with a number of short line segments within a given allowable error for curvilinear interpolation control. FIG. 1 of the accompanying drawings is illustrative of such curvilinear interpolation control. A given curve Li is approximated by line segments P0 P1, P1 P2, P2 P3, P3 P4, P4 P5. It is here assumed that maximum errors between the line segments and the ideal curve Li are t1, t2, t3, t4, t5, respectively, and an allowable error (tolerance) is τ. It is desirable for curvilinear interpolation that all of these maximum errors t1, t2, t3, t4, t5 be smaller than the tolerance.

Two methods which have conventionally been employed for the curvilinear interpolation will be described.

FIG. 2 illustrates the first method. On the ideal curve Li, from a starting point P0, a group of points P1 to P6 are spaced at constant intervals. First, a maximum error t1 between a line segment P0 P1 and the curve Li is compared with a tolerance τ. Since t1 ≦τ, the point P1 is skipped. Then, the points P1 to P6 are successively checked, and it is assumed that t6 >τ is achieved for the first time for the line segment P0 P6. An interpolation point next to the point P0 is then determined as the point P5, which is one point prior to the point P6. Then, the point P5 is set as a new starting point. In this manner, successive interpolated line segments are determined. This method is capable of effecting polygonally approximated interpolation in which the tolerance τ is ensured. However, it is disadvantageous in that the time required for calculation is long since a number of points must be successively checked, and it is not easy for the operator to determine how close the points should be established.

According to the second method, interpolating points are determined using the curvature of a given curve as shown in FIGS. 3 through 6. FIG. 3 is illustrative of the relationship between the radius of curvature τ of a curve Li having the form of an arc, the length d of an approximating line segment, and points where the curve Li is divided by the approximating line. It is assumed that the point C is the center of curvature of the arc, the line segment P1 P2 is the approximating line segment, and the points of intersection between a bisector of the angle ∠P1 CP2 and the curve Li and between this bisector and the line segment P1 P2 are PA and PB, respectively. At this time,

(P1 P2 /2)2 +(PB C)2 =(P1 C)2

Since P1 P2 =d, PB C=ρ-t, P1 C=ρ,

t2 -2ρt+d2 /4=0 (1)

and ##EQU1## Equation (2) gives a maximum line segment length ΔS at the time of approximating the arc having the radius ρ of curvature with a line segment when the tolerance τ is given. That is, ##EQU2## The tolerance is ensured if the line segment has a length of ΔS or smaller.

In general, in the second method a given curve is considered as being composed of a succession of arcs including an arc having an infinite radius of curvature, and such a curve is subjected to linear interpolation. For the sake of simplicity, the second method will be described with reference to curves composed only of a straight line and curves on a two-dimensional plane.

FIG. 4 is explanatory of interpolation of a curve Li made up of an arc having a center of curvature C1 and a radius of curvature ρ1 and an arc having a center of curvature C2 and a radius of curvature ρ2, the arcs blending smoothly with each other at a point P5. From equation (3), the length ΔS1 of a line segment for the arc having the radius of curvature ρ1 is ΔS1 =2√2ρ1 τ-τ2, and the length ΔS2 of a line segment for the arc having the radius of curvature ρ2 is ΔS2 =2√2ρ2 τ-τ2 ĚP1 P2, P2 P3, P3 P4 and P4 P5 are approximated by the line segment having the length ΔS1, and P5 P6, P6 P7 and P7 P8 are approximated by the line segment having the length ΔS2, thus achieving linear interpolation in which the tolerance is ensured.

This method, however, has the following shortcoming: In FIG. 5, an arc having a radius of curvature ρ1 and an arc having a radius of curvature ρ2 are joined at a point PC. Interpolating points are determined from a starting point P0 by line segments P0 P1, P1 P2, P2 P3, P3 P4 and P4 P5 having the length S1 =2√2ρ1 τ-τ2. The length of a line segment at a point P5 is also ΔS1 since P5 PC is an arc having the radius of curvature ρ1. If a next point P6 is determined as being on an arc having the radius of curvature ρ2, then a curve portion P5 PC P6 is approximated by a line segment P5 P6 for which no tolerance τ is ensured.

FIG. 6 is illustrative of interpolation of a curve Li composed of a straight line joined smoothly at a point PC with an arc having a center of curvature C and a radius of curvature ρ. The length of a line segment which approximates the straight line is infinite from the equation (3). Therefore, a maximum line segment length lL is established, and the operator is allowed to select such a maximum line segment length. However, it is troublesome for the operator to ascertain the setting of the maximum line segment length lL and hence inconvenient to handle the same. Where the straight line is approximated with line segments P1 P2, P2 P3 and P3 P4 having the length lL, a next point P4 is determined with the length lL since the point P4 is on the straight line, with the result that no tolerance τ is ensured. This is because the arc has to be approximated with line segments having a length of lS or smaller calculated by lS =2√2ρτ-τ2 with lL >lS.

As discussed above, the second method has a shorter time for calculation than the first method and lends itself to high-speed processing, but is incapable of performing accurate curvilinear interpolation unless the above deficiencies are eliminated.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a system and method for effecting curvilinear interpolation at a high speed with a high accuracy.

In accordance with the invention, a curve is expressed by a vector:

P(S)=[x(S), y(S), z(S)] (4)

using a curvelength S. Generally, a minute portion of a given curve is considered as an arc having a radius of curvature given by: ##EQU3## and the curve is considered as a succession of such minute arcs. Designated by ρ is the radius of curvature, and its reciprocal:

K=1/ρ (6)

called the curvature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrative of conventional interpolation of a curve through linear interpolation;

FIGS. 2 through 6 are diagrams explanatory of conventional interpolation methods and their shortcomings;

FIG. 7 is a block diagram of a system according to the present invention;

FIG. 8 is a diagram showing memory regions;

FIG. 9 is a block diagram of a control circuit in the system of FIG. 7;

FIG. 10 is a flowchart of successive steps of operation of the system of the invention;

FIG. 11 is a diagram showing the relationship between a tolerance τ and a radius of curvature ρ;

FIGS. 12A, 12B and 13A, 13B are diagrams illustrative of operation of the system of the invention;

FIGS. 14 and 15 are diagrams explanatory of advantages of the present invention in comparison with the conventional process; and

FIGS. 16A and 16B are a set of diagrams illustrative of search lengths lS.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Designated in the drawings at 1 is a data input unit, 2 a memory, 3 a control circuit, 4 a servo circuit for driving various axes, and 5 a machine tool.

Identical or corresponding reference characters denote identical or corresponding parts throughout the views.

FIG. 7 is a block diagram of a curvilinear interpolation system according to a preferred embodiment of the present invention. The system includes a data input unit 1 through which the operator enters data items on the shape of a curve, a tolerance τ, and a search length lS, a memory 2 for storing the data on the shape of the curve, the tolerance τ, and the search length lS in the pattern shown in FIG. 8 and also storing data used by a control circuit, a control circuit 3 for issuing coordinate command values to a servo circuit 4 for driving various axes, the servo circuit 4 controlling a machine tool 5 to reach the applied command values. The control circuit 3 is implemented with a microprocessor including operative elements as shown in FIG. 9. In FIG. 9, the control circuit 3 is composed of an element 3a supplied with the length ΔS of a next approximating line segment for issuing a current curve length S, an element 3b for calculating a radius of curvature K1 in the curve length S, an element 3c for calculating a radius of curvature K2 in the curve length S+lS (search length lS), an element 3d for comparing the radii of curvature K1 and K2 and setting a larger radius of curvature to KS, an element 3e for comparing the value of the radius of curvature KS with a limit radius of curvature KL which determines whether the portion having the radius of curvature KS is a straight line or an arc, an element 3f for calculating a line segment length S when the current curve portion is determined as an arc, an element 3g for calculating a line segment length S when the current curve portion is determined as a straight line, and an output unit 3h for calculating coordinate values of the curve from the curve length S and issuing them as X, Y, Z command values to the servo circuit 4.

Operation of the system thus constructed according to the present invention will be described with reference to the flowchart of FIG. 10. First, the curve length S is initialized in a step (1) by enabling the element 3a to set S=0. Then, the limit curvature KL is set in a step (2). As described below, the search length lS is employed to check the condition of the curve in units of this length, and is appropriately selected by the operator dependent on the condition of the curve. No line segment is allowed which has a length in excess of the unit search length. If the limit radius of curvature KL is established from the equation (1) as: ##EQU4## then the curve may be regarded as an arc when the radius of curvature is greater than KL and as a straight line when the radius of curvature is smaller than KL. When regarded as a straight line, the maximum allowable error is smaller than the tolerance if the line segment length is lS. Then, a step (3) determines whether the curve is finished or not. If the curve length S exceeds a certain value, then the program ends, and if not, the program goes to a next step. The element 3h calculates the coordinate values of the current curve from the current curve length S and issues them as command values to the servo circuit in a step (4). A step (5) calculates a radius of curvature K1 in the curve portion having the current curve length S, and a step (6) calculates a radius of curvature K2 in the curve portion having the curve length S+lS which is spaced the search length lS from the current curve portion. A step (7) compares the radii of curvature K1 and K2 and selects the larger one, and a step (7) sets the selected radius of curvature as KS. The steps (7) and (8) will be described in greater detail below.

FIG. 11 is illustrative of the relationship between the radius of curvature of the curve and the tolerance at the time the line segment length ΔS is constant. FIG. 11 shows that for curves having radii of curvature of ρL and larger, the tolerance τ, is ensured if they are approximated by the line segment length lS.

FIG. 12A is explanatory of the case where the curvature of a curve is expressed by a decreasing function (the radius of curvature is an increasing function). The curve Li in FIG. 12A has a curvature gradually decreasing in the direction of the arrow. FIG. 12B is a graph showing the decreasing curvature. The curve has curvatures K1 and K2 at a starting point P0 and a point Pn spaced the search length lS from the starting point P0 along the curve, with K1 >K2. The radii of curvature ρ1 and ρ2 have the relation ρ1 <ρ2. The tolerance τ can completely be ensured by approximating a partial curve P0 Pn with a line segment having the length of ΔS=2√2ρ1 τ-τ2 from the equation (3) at the starting point P0. The curve is expressed by a length parameter S, and if the parameter S of P0 is S1 , then a next interpolating point is determined as S1 +ΔS. If this point is determined as P1, then it is clear that P0 P1 <ΔS and the tolerance τ can be ensured by employing the point P1 as an interpolating point.

FIG. 13A is explanatory of the case where the curvature of a curve is expressed by an increasing function (the radius of curvature is a decreasing function). The curve Li in FIG. 13A has a curvature gradually increasing in the direction of the arrow. FIG. 13B is a graph showing the decreasing curvature. The curve has curvatures K1 and K2 at a starting point P0 and a point Pn spaced the search length lS from the starting point P0 along the curve, with K1 >K2. The radii of curvature ρ1 and ρ2 have the relation ρ1 <ρ2. The tolerance can completely be ensured by employing the curvature and the radius of curvature at a point Pl, establishing the line segment length ΔS as ΔS=2√2ρ2 τ-τ2 and determining a line segment from the point P0. The curve is expressed by a length parameter S, and if the parameter S of P0 is S1, then a next interpolating point is determined as S1 +ΔS. If this point is determined as p1, then it is clear that P0 P1 <ΔS, and the tolerance τ can be ensured by employing the point P1 as an interpolating point.

Where the curvature of the curve is constant, ΔS=2√2ρ1 τ-τ2 is employed. However, it is apparent that either line segment length may be used.

As described above, in the steps (7) and (8) in FIG. 10, the radii of curvature K1 and K2 are compared, the larger one selected, and the selected radius of curvature established as KS.

In a step (10), a limit radius of curvature KL is compared with KS to determine whether the curve to be interpolated is regarded as an arc or a straight line. If regarded as an arc, then a step (11) calculates S from the equation (3). If regarded as a straight line, then a step (12) equalizes ΔS with lS (search length). Then, a step (13) increments the curve length by ΔS, and the program goes back to the step (3). The curve given by the foregoing process can be interpolated with the tolerance τ ensured.

FIG. 14 is explanatory of the present invention in comparison with FIG. 5, and shows a curve composed of an arc having a radius of curvature ρ1 and an arc having a radius of curvature ρ2, the arcs being joined at a point PC. First, curvatures K1 and K2 at a starting point P0 and a point on the curve spaced the search length lS from the point P0 are calculated. Since K1 =K2 =1/ρ1, a line segment P0 P1 of S≦2√2ρ1 τ-τ2 is obtained. Likewise, line segments P1 P2, P2 P3, P3 P4 and P4 P5 are obtained. Curvatures K1, K2 at the point P5 and a point on the curve spaced the search length lS from the point P5 are calculated, and K1 <K2 since the latter point is positioned on the arc having the radius of curvature ρ2. At this time, a line segment P5 P6 having a length S≦2√2ρ2 τ-τ2 is obtained. Thus, the shortcoming explained with reference to FIG. 5 is eliminated.

FIG. 15 is illustrative of operation of the system of the invention in comparison with FIG. 6. That is, FIG. 15 is explanatory of interpolation of a curve Li composed of a straight line and an arc having a center of curvature C and a radius of curvature ρ, the straight line blending smoothly into the arc at a point PC. Curvatures K1 and K2 at a point P1 and a point P2 on the curve Li spaced the search length lS from the point P1 are calculated, and both K1 and K2 are found here to be zero. Since it is apparent that K1 <KL =2√2ρS τ-τ2 the line segment length is equalized with the search length lS, and the point P2 is determined as an interpolating point. Likewise, points P3 and P4 are obtained. Then, curvatures K1 and K2 at the point P4 and a point on the curve Li spaced the search length lS from the point P4 are calculated, and K1 <K2. If K2 <KL then a point p5 is obtained from ΔS=2√2ρτ-τ2. Therefore, the drawback described with reference to FIG. 6 is eliminated.

The curvilinear interpolation system according to the present invention requires the operator to specify a search length as a unit for checking the condition of a curve. The system of the invention is advantageous in that the search length can be handled with greater ease than would be the case with the conventional system in which the operator is required to give a maximum line segment length.

The search length will now be described. When a curve Li of FIG. 16A is checked from the point P1 with a search length lS, a "curved portion" of the curve Li is found at the arrow. Therefore, the entire curve Li is approximated with line segments P1, P2, . . . , P13 with errors smaller than a desired tolerance. When checking a line Li of FIG. 16B with the same search length lS, no "curved portion" is found at the arrow. Where this search length lS is used, no error below the desired tolerance is ensured. In this case, a smaller search length lS ' is established to ensure an error below the tolerance.

Although the curvilinear interpolation system of the the present invention has been described with respect to two-dimensional coordinates, the invention is also applicable to any desired curve in three-dimensional space.

In accordance with the curvilinear interpolation system and method of the invention, a curvature K1 is calculated at a current curve interpolating point Pp and a curvature K2 is calculated a point spaced a given search length lS from the point Pp along the curve, the calculated curvatures are compared, and the respective segment of the curve apportioned with a line segment therealong to determine a next interpolating point. The inventive system and method is thus capable of carrying out curvilinear interpolation at a high speed with a high accuracy through a simple arrangement.